Galaxy Mergers

Over the years evidence has mounted for a significant mode of galaxy evolution via mergers. This process links gas-rich, disk galaxies; starbursting galaxies; active galactic nuclei (AGN); post-starburst galaxies; and gas-poor, dynamically hot, elliptical galaxies, as objects representing different phases of major galaxy mergers. Hydrodynamic simulations of galaxy mergers predict that as the galaxies coalesce, gravitational forces funnel gas toward the center, which provides a fuel reservoir to feed the central supermassive black hole and to form large numbers of stars in a nuclear starburst. The AGN activity may be optically obscured for a time, until feedback due to either supernova winds or the AGN accretion blow out much of the surrounding gas in an outflow. At this point the accreting black hole is exposed as an observable optical quasar. After the blowout, remaining nuclear gas is soon used up, ending AGN activity and quickly quenching the nuclear starburst in the galaxy. The galaxy then quiescently fades to a red elliptical.

The post-starburst (or E+A) phase is particularly interesting because nearly every galaxy that evolves from an actively star-forming phase to a quiescent, “red and dead” phase must pass through it. In essence, the E+A phase is a sort of galaxy evolution “bottleneck” that indicates that a galaxy is actively evolving through several important physical transitions. Careful study of E+A galaxies could in principle disentangle some of the many paths of galaxy evolution that lead to it – including major merger parameters, gas consumption, and AGN formation and duty cycles.

UW researchers are putting observational constraints on the timing of the phases of galaxy mergers by studying the transitional objects – those near the end of or after the starburst phase. The study is based on a sample of galaxies identified from the SDSS that show a post-starburst signature in their spectra and have radio properties that indicate the presence of buried AGN. Ongoing observational campaigns in multiple wavelength regimes provide the data from which timeline information is being extracted.

We use optical spectra that are spatially distributed across the galaxies from the WIYN and SALT telescopes to study the stellar populations. How long ago did a burst of star formation happen? How long did it last? In which parts of the galaxy did it occur? What stellar mass was formed during the burst?

We use high resolution near infrared images from the WHIRC camera on WIYN to determine from spatially distributed galaxy colors whether large amounts of dust could be obscuring ongoing star formation or AGN activity in the galaxies.

We use radio fluxes observed over a range of frequencies with the VLA and GMRT radio interferometers to estimate how long the radio emission from the AGN has been active. Does the timing of the AGN match that of the starburst or not?